Ozone variability in the marine boundary layer of the eastern Mediterranean based on 7-year observations

[1] A 7-year time series ( 1997 - 2004) of surface ozone at Finokalia, Crete, in the eastern Mediterranean, was analyzed to investigate the mechanisms that control ozone levels and variability in the area. Transport from the European continent was identified as the main mechanism that controls ozone levels in the eastern Mediterranean, especially during summer when ozone presents a summer maximum ( July) of 58 +/- 10 ppbv. Radon 222 has been proved a useful tool for the verification of the continental origin of ozone. The role of local photochemistry and pollution becomes important under western flow and stagnant wind conditions. For the whole period, a profound ozone-decreasing trend was also observed with a decline of 1.64 +/- 0.15 ppbv yr(-1), or 3.1% per year. The sharp decline of ozone during the first 5 years (i.e., 3.4 +/- 0.2 ppbv yr(-1) or 5.6% per year for 1998 2002) has been succeeded by an abrupt increase in 2003 ( to the 1999 ozone levels), followed by a return to the "regular'' ozone declining levels in 2004. The rates of the decline were higher for the spring and summer concentrations. In parallel with the ozone decline, a shift of the maximum ozone concentrations from summer to spring, attributed to a continuous decrease of the summer ozone concentrations, was also observed, with the year 2002 presenting a clear spring maximum. The decline of ozone and the shift of its maximum to spring could be related to the reduction of ozone precursors that occurred both in western/central and eastern European countries. The severe weather phenomena that influenced mainly central Europe in summer 2002 have also affected ozone measurements at Finokalia, since the induced meteorological disturbance caused the prevalence of NW winds instead of the dominant NE flow.